Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHODS FOR MEASURING CONDUCTIVITY OF NON-AQUEOUS
LIQUIDS AT VARIABLE TEMPERATURES AND APPLIED VOLTAGES
Field of the Invention
The invention relates generally to methods for measuring the conductivity of
non-
aqueous liquids, such as crude oils and crude oil blends.
Description of the Related Art
The conductivity of non-aqueous liquids, such as crude oils is an important
factor
for determining how such liquids must be handled, stored and processed. If,
for example, a
certain liquid has high conductivity at a particular temperature, it would be
important to
store that liquid in a manner wherein it is protected from potential
electrical discharge.
Currently, there are difficulties measuring conductivity of non-aqueous
liquids in a safe
and effective manner. For example, samples can evaporate at the temperatures
used during
testing. Furthermore, at specific temperatures, the conductivity of crude oil
can be grossly
miscalculated due to polarization effects, which are typically seen upon
application of a
DC (direct current) field.
Summary of the Invention
The invention provides methods for effectively measuring conductivity of non-
aqueous liquid samples or sample blends. In particularly preferred
applications, the
methods are useful for determining of the conductivity of samples of crude
oil, blends of
crude oil or other non-aqueous liquids at process temperatures and pressures.
An exemplary method of measurement is described wherein a sample is placed
within the testing vessel of a conductivity cell. First and second electrodes
are operably
associated with the conductivity cell so that a voltage source can apply a
voltage across the
sample. The conductivity cell is preferably disposed within a heater block
which is
associated with a heating controller. The heating controller controls the
temperature of the
heating block and conductivity cell. Preferably, the heating controller is a
cascade
feedback controller which optimally adjusts temperature in a manner which will
attain a
desired temperature in a rapid fashion.
In accordance with a preferred method of measurement, the sample is
pressurized
using a pressure control system which subjects the sample to a pressure
blanket of inert
gas. Also in preferred embodiments, the pressure blanket is at a pressure that
is
significantly greater than the vapor pressure of the liquid sample.
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After the liquid sample has been brought to a desired pressure and heated to a
desired temperature, a voltage or series of voltages is/are applied across the
first and
second electrodes of the conductivity cell by the voltage source. The power
source is
preferably operable to apply varying levels of voltage to the sample for
certain lengths of
time. In particular embodiments, DC voltage is applied in sequences of varying
levels over
short intervals of time, such as 1 second or less. The RTD immersed within the
sample
detects the current induced through the liquid sample by the application of
voltage.
In accordance with an exemplary method of determining conductivity of a liquid
sample, a liquid sample is placed within the testing vessel and an RTD
(resistance
to temperature detector) is immersed into the liquid sample, thereby
forming a conductivity
cell. The conductivity cell is retained within a heater block and further
connected to the
pressure control system. The conductivity cell is maintained under pressure
using an inert
gas pressure blanket, after which the heater block is heated to a desired
temperature by the
heating controller. A programmable data acquisition system is used to specify
the
temperatures at which voltage needs to be applied, the type of voltage (DC or
AC) that
needs to be applied, the desired time duration for which voltage should be
applied and the
sequence of voltages (usually ranging from 1V-100V) which need to be applied.
When the
desired voltage is applied to the liquid sample in the annular region of the
conductivity
cell, a conductivity probe measures the current within the sample as voltage
is applied
across it. During testing, the temperature of and pressure applied to the
liquid sample may
be varied, which allows measurement of conductivity in a variety of
conditions. In
accordance with some embodiments, the voltage (type, magnitude and time
duration of
application) which is applied to the liquid sample can be adjusted during
testing.
Measuring conductivity of crude oil is non-trivial, especially at elevated
temperatures where lighter blends of the crude oil tend to evaporate and in
turn change the
composition (and resultant conductivity) of the crude oil blend. The
evaporation potential
is reduced by application of a pressure blanket that is maintained at a
pressure that is
sufficiently higher than the vapor pressure of the liquid sample to prevent
any of the
sample from escaping via evaporation. A pressure transducer attached to the
conductivity
cell and the data acquisition system helps monitor the pressure of the sample.
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Brief Description of the Drawings
For a thorough understanding of the present invention, reference is made to
the
following detailed description of the preferred embodiments, taken in
conjunction with the
accompanying drawings, wherein like reference numerals designate like or
similar
elements throughout the several figures of the drawings and wherein:
Figure 1 is a side, cross-sectional view of an exemplary testing vessel
portion of a
conductivity cell constructed in accordance with the present invention.
Figure 2 is a side, exterior view of the testing vessel shown in Figure 1, now
turned
90 degrees.
Figure 3 is a top view of the testing vessel.
Figure 4 is side, cross-sectional view of an exemplary conductivity cell
containing a
liquid sample whose conductivity is to be tested.
Figure 5 is a side view of an exemplary heater block containing the
conductivity
cell shown in Figure 1-3.
Figure 6 depicts a conductivity meter wherein the conductivity cell and heater
block are contained within an outer pressure vessel.
Figure 7 is a diagram depicting steps of an exemplary method for measuring
conductivity of a sample of non-aqueous liquid.
Detailed Description of the Preferred Embodiments
Figures 1-6 depict features of an exemplary conductivity meter 10 constructed
in
accordance with the present invention. The conductivity meter 10 includes a
conductivity
cell 12, portions of which are illustrated in Figures 1-4. Figures 1-3 depict
a cylindrical
testing vessel 14 which defines a liquid sample chamber 16 within. An
electrode cavity 18
is formed on the exterior radial surface of the testing vessel 14. The cross-
sectional view
of Figure 1 shows that the electrode cavity 18 preferably has an angled lower
portion 19
which extends radially inwardly into the body of the testing vessel 14. As
Figures 2 and 3
depict, the upper surface 20 of the testing vessel 14 preferably includes
threaded openings
22 and an 0-ring groove 24. A lateral port 26 is disposed through the testing
vessel 14 and
permits a liquid sample within the testing vessel 14 to be exposed to external
pressure.
Figure 4 depicts the conductivity cell 12 assembled with a non-aqueous liquid
sample 28 within the liquid sample chamber 16 and a cap 30 which has been
secured to the
upper surface 20 of the testing vessel 14. 0-ring 32 is seated in 0-ring
groove 24 and
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provides a fluid seal between the testing vessel 14 and cap 30. Threaded
connectors 34
secure the cap 30 to the testing vessel 14. The cap 30 has a central opening
36 formed
therein. It is noted that the testing vessel 14 and cap 30 are preferably
formed of stainless
steel. A first conductive electrode 38 is shown seated within the electrode
cavity 18. In
accordance with preferred embodiments, the non-aqueous liquid making up the
sample 28
is crude oil or another hydrocarbon liquid. Also, it is noted that use of the
term "non-
aqueous" in this description does not require strict absence of water. Rather,
"non-
aqueous" liquids, as referred to herein, will refer to liquids which contain
either no water or
small percentages of water which do not appreciably affect the conductivity of
the liquid.
A resistance temperature detector (RTD) 40 is inserted through the central
opening
36 and into the liquid sample 28. The resistance temperature detector 40
presents a
conductive distal end 42 which extends into the liquid sample 28 and is
operable to
function as a second electrode for the conductivity cell 12. In addition, the
resistance
temperature detector 40 also has the capability of measuring current flow
through the
liquid sample 28 passing between the first electrode 38 and the second
electrode provided
by the distal end 42. The distal end 42 therefore also contains a conductivity
probe.
Preferably also, the resistance temperature detector 40 is capable of
detecting the
temperature of the liquid sample 28.
Figure 5 illustrates the conductivity cell 12 now having been disposed within
a
heater block 44. The conductivity cell 10 is disposed within a conductivity
cell sleeve 46
so that the testing vessel 14 is largely disposed within the interior chamber
48 of the heater
block 44. A heater line 50 provides heated media to the interior chamber 48 to
heat the
liquid sample 28. A thermocouple 52 is associated with the heater block 44 to
monitor the
temperature at which the interior chamber 48 and conductivity cell 10 are
maintained.
Figure 6 illustrates a substantially complete conductivity meter 60 in
accordance
with the present invention wherein the heater block 44 is associated with a
pressure control
system 61 that is used to provide a pressure blanket of inert gas during
testing. The heater
block 44 is shown contained within a pressure vessel 62 which defines an
interior pressure
chamber 64. A relief valve 66 is preferably located on the top side of the
pressure vessel
62 and preferably can be selectively opened and closed to permit
depressurization of
excess gas within the pressure chamber 64. A pressurized fluid source 68 is
located
proximate the pressure vessel 62 to supply a pressurized fluid to the pressure
chamber 64
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via conduit 70. The pressurized fluid is preferably an inert gas, such as
ultrapure argon or
nitrogen. If necessary, the pressurized fluid source 68 may include a fluid
pump to
generate desired pressures within the pressure chamber 64 as well as suitable
valves to
control the flow of pressurized fluid into the pressure chamber 64. A pressure
transducer
69 is associated with the conductivity cell 10 and provides a signal
indicative of measured
pressure to pressure gauge 71, thereby enabling a user to control pressure
within the
pressure vessel 62 to achieve a desired pressure. The pressurized fluid source
68 is used to
maintain the conductivity cell 10 and liquid sample 28 at a desired pressure.
In addition,
the pressurized fluid source 68 provides a pressure blanket which prevents or
limits
to
evaporation of the liquid sample 28 during testing. When an inert gas is used
for the
pressure blanket, an inert, oxygen-free environment is provided which prevents
potential
ignition of the sample during testing.
A heating controller 72 is operably interconnected with the thermocouple 52
and is
operable to supply heated medium via conduit 50 into the interior chamber 48
of the heater
block 44. The heating controller 72 maintains the temperature within the
interior chamber
48 in accordance with temperature feedback provided by the thermocouple 52. In
preferred embodiments, the heating controller 72 is a cascade feedback
controller, of a type
known in the art, which can provide optimized heating by attaining a desired
temperature
in a rapid manner.
The first and second electrodes 38, 40 are operably associated with a voltage
power
source 74 which are capable of supplying a voltage potential across the
electrodes 38, 40 of
the conductivity cell 10. The applied voltage might be an AC or a DC voltage.
In
preferred embodiments, the voltage power source 74 is capable of applying a
constant DC
voltage, such as 3V DC, for a predetermined length of time. In further
preferred
embodiments, the voltage power source 74 is capable of applying a variable
range of DC
voltages to the electrodes 38, 40. For example, the voltage power source 74
might be able
to apply voltages within a range from 0-1000 V DC and be programmable to
change
between voltage levels in accordance with a predetermined scheme. A
conductivity/resistance detector 76 is also operably associated with the
conductivity probe
of distal end 42 and is operable to detect the conductivity between the
electrodes 38, 40
when a voltage is applied across them. The
voltage source 74 and the
conductivity/resistance detector 76 may be combined in a single device, as
Fig. 6
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illustrates. A suitable voltage source and conductivity/resistance detector
for use as the
components 74, 76 is a Keithley brand picoammeter, which is available
commercially from
Keithley Instruments, Inc. of Cleveland, Ohio. In particular embodiments, the
voltage
source 74 applies DC voltage in sequences of varying levels of voltage over
short intervals
of time (typically one second or less intervals). The application of pulsed DC
field
sequences, along short time intervals, is useful to overcome the effects of
polarization,
which could result in miscalculation of conductivity for a sample.
In accordance with particular embodiments, the conductivity meter 60 includes
a
cold gun 78 for rapidly cooling down the heater block 44 and conductivity cell
10 as a
safety measure.
The conductivity meter 60 is useful to measure the conductivity of a sample 28
of
non-aqueous liquid, such as crude oil. In operation, a sample 28 is placed
within the liquid
sample chamber 16 of the testing vessel 14 and cap 30 is affixed to the
testing vessel 14.
The resistance temperature detector 40 is inserted into the liquid sample 28
and the first
electrode 38 is inserted into the electrode cavity 18. The conductivity cell
10 is then
inserted into the conductivity cell sleeve 46 of the heater block 44 and the
meter 60 further
assembled in the manner depicted in Figure 6. Next, the heating controller 72
is actuated
to heat the heater block 44 and conductivity cell 10 to a predetermined
temperature. The
pressurized fluid source 68 adjusts the pressure within the pressure chamber
64 to a
predetermined pressure level. Thereafter, a desired voltage level or levels
is/are applied to
the conductivity cell 10 by the voltage source 74 as conductivity is measured
by the
conductivity detector 76. The conductivity meter 60 allows measurement of
conductivity
over a range of temperatures and pressures as well as an over a range of
applied voltages.
Figure 7 depicts exemplary method steps for a method of measuring conductivity
of
a sample 28 of non-aqueous liquid in accordance with the present invention. In
accordance
with step 90 of the method, a sample 28 of non-aqueous liquid is disposed
within a
conductivity cell 10. In step 92, the conductivity cell 10 and sample 28 are
pressurized to a
predetermined pressure. Typically, this would be done by operation of the
pressure control
system 61 which provides a pressure blanket for the sample 28. In step 94, the
conductivity cell 10 and sample 28 are then heated to a predetermined
temperature by the
heating controller 72. In step 96, a voltage is applied across the sample 28
by the voltage
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source 74. Conductivity of the sample 28 is then detected by the conductivity
meter 60 as
the voltage is applied in step 98.
Those of skill in the art will recognize that numerous modifications and
changes
may be made to the exemplary designs and embodiments described herein and that
the
invention is limited only by the claims that follow and any equivalents
thereof.
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